Many radio receivers are capable of receiving broadcast signals at a plurality of different selectable frequencies. The radio will ordinarily demodulate the signal in order to extract the information it carries and make it audible to the operator of the radio. In most commercial broadcast spectrums, the frequency at which such signals may be broadcast are ordinarily specified by a government agency. For example, in the AM broadcast spectrum in North America, stations can only broadcast on frequencies located at 10 kHz increments.
This does not mean, however, that a viable broadcast signal can be found at every 10 kHz increment in the AM range in every broadcast area. Instead, some of these channels will ordinarily remain unassigned in certain areas, and such unassigned channels will generally be void of useful transmissions.
Many radio receivers include a channel scanning mechanism that, when activated by the operator, will cause a station tuning mechanism to increment in frequency until a broadcast station can be located. Since all possible broadcast frequencies may not be used in a given area, such a channel scanning mechanism cannot simply stop at each potentially assigned frequency. Further, such a mechanism cannot simply monitor for energy at potentially assigned frequencies because nonuseful transmissions, such as noise, may be present and be falsely interpreted as a viable broadcast signal.
One prior art solution having particular relevance in an AM stereo radio receiver can be seen in FIG. 1. Many AM stereo receivers include an in phase multiplier (A) and a quadrature multiplier (B) that both receive an intermediate frequency signal defined as Acos(Wit+.phi.) (where "A" constitutes an amplitude factor, "wi" comprises the frequency of the IF section, and ".phi." includes pertinent stereo information). The output of the quadrature multiplier (B) drives a voltage controlled oscillator (C) that, when locked with respect to the incoming signal, provides two outputs. The first output comprises sin(Wit) as supplied to the quadrature multiplier (B) and the second output comprises cos(Wit) as supplied to the in phase multiplier (A). The in phase multiplier (A) multiplies the incoming signal by the VCO (C) output to provide an Acos.phi. output signal.
When a broadcast station has been locked on to as described above, this output signal from the in phase multiplier (A) will generally include only positive polarity components. Therefore, by providing a capacitor (D) to integrate this signal over time and thereby obtain an average, the output of the in phase multiplier (A) can be monitored to determine whether a broadcast station has indeed been tuned.
Unfortunately, however, the above described prior art approach does not provide a completely adequate solution in all settings.
This prior art approach indicates only the presence or absence of a broadcast signal, and provides no indication regarding the potential quality of the received signal. Therefore, this approach might cause a channel scanning mechanism to stop on an unacceptably noisy station or on a distant station that may drift in and out of audible range and therefore prove an unsatisfying selection.
There therefore exists a need for a broadcast signal detected indication system that can provide some measure of signal quality.